1. Field of the Invention
The present invention relates generally to an apparatus for imaging and displaying a blood vessel, and more particularly to an apparatus for imaging and distinguishing an artery from a vein or vice versa.
2. Description of the Related Art
In clinical diagnosis, there have been wide demands for imaging and discriminating an artery from a vein or vice versa. For example, arteriosclerosis generally arises from a peripheral portion. Therefore, if the inside diameter image of the artery in this peripheral portion can be discriminated from a venous image and imaged, it can be utilized as diagnostic information with respect to arteriosclerosis.
As an apparatus for imaging and displaying a blood vessel, an X-ray blood vessel contrast photographing apparatus has hitherto been widely known. However, this X-ray blood vessel contrast photographing places a great burden on a subject and the execution thereof usually requires admission to a hospital, so there is a problem that it is difficult to easily perform the X-ray blood vessel contrast photographing on an outpatient.
In contrast to this, a technique of imaging a part of a living organism by light fluoroscopy has also been proposed as indicated in Medical Society Journal of Japan, BME Vol. 8, No. 5, 1994, pp. 41-50. In this imaging technique by light fluoroscopy, however, it is extremely difficult to clearly image and distinguish an artery-from a vein or vice versa.
The present invention has been made in view of the aforementioned circumstances. Accordingly, it is an object of the present invention to provide an apparatus which places a lower burden on a subject and is capable of imaging and distinguishing an artery from a vein or vice versa.
A blood vessel imaging apparatus according to the present invention applies optical heterodyne detection to imaging such that high space resolution is ensured with respect to a living organism which is a scattering medium, and distinguishes an artery and vein by taking advantage of a difference in light absorption characteristic between oxyhemoglobin and deoxyhemoglobin in the blood.
More specifically, the blood vessel imaging apparatus according to the present invention comprises:
light source means for emitting a first measuring light beam and a second measuring light beam differing from the first measuring light beam, the first measuring light beam having a wavelength equal to a wavelength at an isosbestic point between oxyhemoglobin and deoxyhemoglobin in the blood of a living organism;
an incident optics system for causing the first measuring light beam and the second measuring light beam to be incident on the same part of the living organism;
scanner means for scanning the living organism with the first measuring light beam and the second measuring light beam;
a first optical heterodyne detection system equipped with a first optics system for synthesizing the first measuring light beam and a branched first measuring light beam transmitted through the living organism; a first frequency shifter for giving a difference in frequency between the first measuring light beam and the branched first measuring light beam; and first detection means for detecting a first beat component of the synthesized first measuring light beam and outputting a first beat component detection signal;
a second optical heterodyne detection system equipped with a second optics system for synthesizing the second measuring light beam and a branched second measuring light beam transmitted through the living organism; a second frequency shifter for giving a difference in frequency between the second measuring light beam and the branched second measuring light beam; and second detection means for detecting a second beat component of the synthesized second measuring light beam and outputting a second beat component detection signal; and
image signal generation means for generating an image signal, based on a value of the second beat component detection signal normalized by the first beat component detection signal.
In a preferred form of the present invention, the light source means emits a light beam of wavelength xcex1 as the first measuring light beam and emits a light beam of wavelength xcex2 as the second measuring light beam, and when it is assumed that a value of a beat component detection signal related to the measuring light beam of wavelength xcex1 is Ixcex1 and a beat component detection signal related to the measuring light beam of wavelength xcex2 is Ixcex2, the image signal generation means generates the image signal, based on a value of log(Ixcex2/Ixcex1).
The wavelength xcex1 of the first measuring light beam may be 805 nm and the wavelength xcex2 of the second measuring light beam may be 760 nm. Also, the wavelength xcex1 may be 805 nm and the wavelength xcex2 may be 930 nm.
In another preferred form of the present invention, the light source means emits a light beam of wavelength xcex1 as the first measuring light-beam and emits a light beam of wavelength xcex2 and a light beam of wavelength xcex3 as the second measuring light beam, and when a value of a beat component detection signal related to the measuring light beam of wavelength xcex1 is assumed to be Ixcex1, a beat component detection signal related to the measuring light beam of wavelength xcex2 to be Ixcex2, and a beat component detection signal related to the measuring light beam of wavelength xcex3 to be Ixcex3, the image signal generation means generates the image signal, based on a difference between a value of log(Ixcex2/Ixcex1) and a value of log(Ixcex3/Ixcex1).
In the case of employing three kinds of measuring light beams, as described above, the wavelengths xcex1, wavelength xcex2, and the wavelength xcex3 are, for example, 805 nm, 760 nm, and 930 nm.
In still another preferred form of the present invention, the blood vessel imaging apparatus according to the present invention further comprises synchronous detection means for detecting a pulse wave of the artery of the living organism and performing the beat component detection of the first and second measuring light beams in synchronization with a predetermined phase of the pulse wave.
The arterial blood of a living organism includes oxyhemoglobin dominantly, while the venous blood includes deoxyhemoglobin dominantly. FIG. 6 shows the absorption spectra of oxyhemoglobin and deoxyhemoglobin that are light-absorbing materials, along with the spectrum of water that determines the optical characteristics of the tissues of the human body. As shown in the figure, the spectrum of oxyhemoglobin has a characteristic of low absorption on the short wavelength side of the isosbestic point (wavelength 805 nm), while the spectrum of deoxyhemoglobin has a characteristic of low absorption on the long wavelength side of the isosbestic point.
On the other hand, the beat component detection signals, output by the above-mentioned first and second optical heterodyne detection systems, indicate the intensities of only the straight light portion transmitted through the living organism and the scattered light portion close thereto, excluding the influence of scattering of the living organism that is a scattering medium. The value of the beat component detection signal will become greater if absorption of the measuring light beam is less.
Hence, in consideration of the absorption spectra of FIG. 6, consider the case of using a light beam of wavelength xcex1=805 nm equal to the isosbestic point wavelength as the first measuring light beam and using, for example, a light beam of xcex2=760 nm (where the absorption of deoxyhemoglobin is particularly greater with respect to the absorption of oxyhemoglobin) as the second measuring light beam.
If, in the above case, the first and second measuring light beams are transmitted through the venous part in which deoxyhemoglobin is dominantly included, the second beat component detection signal that is output by the second optical heterodyne detection system basically indicates a lesser value because absorption is greater, as compared with the first beat component detection signal that is output by the first optical heterodyne detection system. If, on the other hand, the first and second measuring light beams are transmitted through the arterial part in which oxyhemoglobin is dominantly included, the second beat component detection signal that is output by the second optical heterodyne detection system basically indicates a greater value because absorption is less, as compared with the first beat component detection signal that is output by the first optical heterodyne detection system.
The beat component detection signals that are output by the first and second optical heterodyne detection systems are influenced by light attenuation (absorption and scattering) due to soft tissues or bones other than blood and a change in the amount of blood. However, if the second beat component detection signal output by the second optical heterodyne detection system is normalized based on the first beat component detection signal output by the first optical heterodyne detection system, the normalized value will exclude these major causes of change and accurately indicate a relation in magnitude between both signals based on the above-mentioned difference in absorption characteristic.
Therefore, by generating an image signal on the basis of the aforementioned normalized value, either the arterial part alone or the venous part alone can be imaged. That is, for example, when the values of the first and second beat component detection signals of the first and second optical heterodyne detection systems are assumed to be Ixcex1 and Ixcex2, respectively, the value (Ixcex2/Ixcex1) of the latter normalized by the former will assume a value greater than 1 if the first and second measuring light beams are transmitted through the arterial part and assume a value less than 1 if the first and second measuring light beams are transmitted through the venous part.
Hence, if only a positive value of log(Ixcex2/Ixcex1), obtained for each scanning position by scanning the living organism with the first and second measuring light beams, is converted to an image signal and an image is reproduced by the image signal, then the image will show the arterial part alone. If, on the other hand, only a negative value of log(Ixcex2/Ixcex1) obtained for each scanning position is converted to an image signal and an image is reproduced by the image signal, then the image will show the venous part alone.
It is also possible to image either the arterial part alone or the venous part alone, based on the relation in magnitude between the aforementioned normalized value (Ixcex2/Ixcex1) and threshold value=1.
On the other hand, assume that the beat component detection signal related to the measuring light beam of wavelength xcex1 is Ixcex1, the beat component detection signal related to the measuring light beam of wavelength xcex2 is Ixcex2, and the beat component detection signal related to the measuring light beam of wavelength xcex3 is Ixcex3. When an image signal is generated based on the difference between a value of log(Ixcex2Ixcex1) and a value of log(Ixcex3/Ixcex1), the advantage that the absolute value of the signal becomes greater according to the difference is obtained, as compared with the case of generating an image signal on the basis of either only a value of log(Ixcex2/Ixcex1) or only a value of log(Ixcex3/Ixcex1).